Method for producing paraxylene using steps in a simulated moving bed and steps fractionating via a three-fraction column
阅读说明：本技术 使用模拟移动床中的步骤和经由三馏分塔分馏的步骤来生产对二甲苯的方法 (Method for producing paraxylene using steps in a simulated moving bed and steps fractionating via a three-fraction column ) 是由 I.普雷沃 J.皮古里耶 P-Y.马丁 A.科特 于 2019-06-28 设计创作，主要内容包括：本发明涉及生产高纯度对二甲苯的方法,所述方法包括在SMB中通过吸附分离的单个步骤、随后在第一三馏分蒸馏塔中通过蒸馏分离以产生至少两个提余液的步骤和任选两个异构化步骤,所述方法可以改善芳族环路的对二甲苯总收率并使经济影响最小化。(The present invention relates to a process for producing high purity para-xylene comprising a single step of separation by adsorption in SMB, followed by a step of separation by distillation in a first three-fraction distillation column to produce at least two raffinates and optionally two isomerization steps, which can improve the overall yield of para-xylene of the aromatic loop and minimize economic impact.)
1. a process for producing para-xylene from a feedstock containing xylene, ethylbenzene and C9+ hydrocarbons comprising:
-a single step a of separating the feedstock, carried out in a simulated moving bed at a volume ratio of desorbent to feedstock of from 0.4 to 2.5 in a simulated moving bed separation unit at a temperature of from 20 to 250 ℃ and a pressure of from 1.0 to 2.2MPa, using zeolite as adsorbent and desorbent;
the step a may result in:
-a step B of fractionating by distillation the second fraction resulting from step a in at least one first distillation column (B-C3), said step B being able to produce three fractions:
a first fraction (B2) containing EB, OX and MX,
a second fraction containing OX and MX (B3) and
2. The process according to claim 1, wherein the first fraction (B2) resulting from step B has an EB content which is greater than the EB content of the second fraction (B3) resulting from the first column (B-C3) used in step B.
3. A process according to claim 2 wherein the EB content of the first fraction is at least 1.0% higher than the EB content of the second fraction.
4. The process according to any one of claims 1 to 3, wherein the distillation column used in step B comprises from 30 to 80 theoretical plates.
5. Process according to any one of claims 1 to 4, wherein separation step A also produces a third fraction A22 depleted in EB, containing a mixture of MX, OX and desorbent, said fractions A21 and A22 being fed to said fractionation step B.
6. Process according to any one of claims 1 to 4, wherein the second fraction (B3) is fed to fractionation step B in a second distillation column (B-C4) thereby producing a fraction (B31) free of desorbent, containing MX and OX and a fraction B43 consisting of desorbent.
7. Process according to claim 5, wherein the third fraction (A22) resulting from step A is fed to a second distillation column (B-C4) thereby producing a desorbent-free, MX-and OX-containing fraction (B31) and a desorbent-consisting fraction B43.
8. The process according to claim 7, wherein the third fraction (A22) resulting from step A is introduced into the second distillation column (B-C4) below the lateral injection point of the second fraction (B3) resulting from the first distillation column (B-C3).
9. The process according to any one of the preceding claims, wherein the distillation step B uses a column comprising an inner wall.
10. A process according to any one of the preceding claims, comprising a step C of gas phase isomerisation of the first fraction (B2) containing EB, OX and MX resulting from the fractionation step.
11. Process according to any one of claims 1 to 5, comprising a step D of liquid phase isomerization of a second fraction (B3) containing OX and MX resulting from the first distillation column (B-C3) used in fractionation step B.
12. The process according to any one of claims 6 to 10, comprising a step D of liquid phase isomerization of the OX and MX containing fraction (B31) resulting from the second distillation column (B-C4) used in the fractionation step.
13. The process according to claim 10, wherein the reaction is carried out at a temperature higher than 300 ℃, at a pressure lower than 4.0MPa, for a period of less than 10.0h-1A hydrogen to hydrocarbon molar ratio of less than 10.0 and in the presence of a catalyst comprising at least one zeolite having channels whose pore size is defined by a ring (10MR or 12MR) having 10 or 12 oxygen atoms and at least one metal from group VIII in an amount of from 0.1% to 0.3% by weight.
14. The process according to claim 13, wherein the catalyst used in step C comprises from 1% to 70% by weight of a zeolite of structure type EUO, said zeolite comprising silicon and at least one element T preferably selected from aluminium and boron, said zeolite having a Si/T ratio ranging from 5 to 100.
15. A process according to claim 11 or claim 12 wherein less than 5.0h at a temperature of less than 300 ℃, a pressure of less than 4.0MPa-1Preferably 2.0 to 4.0h-1In the presence of a catalyst comprising at least one zeolite having channels whose pore size is defined by a ring (10MR or 12MR) having 10 or 12 oxygen atoms; preferably, the zeolite is of the ZSM-5 type.
Technical Field
Para-xylene is used primarily for the production of terephthalic acid and polyethylene terephthalate resins, which are used for the production of synthetic textiles, bottles, and more generally plastics.
The present invention relates to a process for producing high purity paraxylene using a specific sequence of steps to achieve high yields of paraxylene.
Background
The use of a separation by adsorption step to produce high purity para-xylene is well known in the art. Industrially, said steps are carried out within a series of "C8-aromatic loop" or "xylene loop" processes. This "C8-aromatic loop" comprises a step of removing heavy compounds (i.e. compounds containing more than 9 carbon atoms, denoted C9+) in a distillation column called "xylene column".
The overhead stream from the column containing the C8-aromatic isomer is then sent to a para-xylene separation process, which is typically a step of separation by adsorption in a simulated moving bed.
The extract obtained at the end of the step of separation by adsorption in a simulated moving bed, which contains para-xylene, is then distilled using an extraction column, then using a toluene column, to obtain para-xylene of high purity.
After the step of removing the desorbent by distillation, the raffinate obtained at the end of the step of separation by adsorption in a simulated moving bed, which is enriched in meta-xylene, ortho-xylene and ethylbenzene, is used in the isomerization step, it being possible to obtain a mixture in which the proportions of xylenes (or ortho-xylene, meta-xylene and para-xylene) are practically in thermodynamic equilibrium and depleted in ethylbenzene. This mixture is fed again to the "xylene column" together with fresh feed for the production of para-xylene.
Disclosure of Invention
Accordingly, the present invention relates to a process for producing para-xylene from a feedstock containing xylene, ethylbenzene and C9+ hydrocarbons, comprising:
-a single step a of separating the feedstock, carried out in a simulated moving bed at a volume ratio of desorbent to feedstock of from 0.4 to 2.5 in a simulated moving bed separation unit at a temperature of from 20 to 250 ℃ and a pressure of from 1.0 to 2.2MPa, using zeolite as adsorbent and desorbent;
the step a may result in:
-a step B of fractionating by distillation the second fraction resulting from step a in at least one first distillation column B-C3, said step B being able to produce three fractions:
Preferably, the first fraction B2 produced from step B has an EB content which is greater than the EB content of the second fraction B3 produced from the first column B-C3 used in step B.
Preferably, the EB content of the first fraction is at least 1.0% higher than the EB content of the second fraction.
Preferably, the distillation column used in step B comprises from 30 to 80 theoretical plates.
In another particular embodiment, the separation step a may also produce a third fraction a22 depleted in EB, containing a mixture of MX, OX and desorbent, said fraction a21 and said fraction a22 being fed to said fractionation step B.
Advantageously, in this embodiment, the third fraction a22 resulting from step a is fed to a second distillation column B-C4, thereby producing a desorbent-free, MX and OX containing fraction B31 and a desorbent consisting fraction B43.
In another particular embodiment, the second fraction B3 is fed to fractionation step B in a second distillation column B-C4, thereby producing a desorbent-free, MX and OX-containing fraction B31 and a desorbent-composed fraction B43.
Advantageously, in this embodiment, the third fraction A22 resulting from step A is introduced into the second distillation column B-C4 below the lateral injection point of the second fraction B3 resulting from the first distillation column B-C3.
Advantageously, the distillation step B uses a column comprising an inner wall.
Advantageously, the process further comprises a step C of gas phase isomerization of the first fraction B2 containing EB, OX and MX resulting from the fractionation step.
Advantageously, the process also comprises a step D of liquid phase isomerization of a second fraction B3 containing OX and MX resulting from the first distillation column B-C3 used in fractionation step B.
Advantageously, the process further comprises a step D of liquid phase isomerization of a OX and MX containing fraction B31 resulting from a second distillation column B-C4 used in the fractionation step.
Advantageously, at a temperature higher than 300 ℃, a pressure lower than 4.0MPa, less than 10.0h-1A hydrogen to hydrocarbon molar ratio of less than 10.0 and in the presence of a catalyst comprising at least one zeolite having channels whose pore size is defined by a ring (10MR or 12MR) having 10 or 12 oxygen atoms and at least one metal from group VIII in an amount of from 0.1% to 0.3% by weight.
Advantageously, the catalyst used in step C comprises from 1% to 70% by weight of a zeolite of structure type EUO comprising silicon and at least one element T preferably selected from aluminium and boron, the zeolite having a Si/T ratio ranging from 5 to 100.
Advantageously, at a temperature lower than 300 ℃, a pressure lower than 4.0MPa, for less than 5.0h-1Preferably 2.0 to 4.0h-1In the presence of a catalyst comprising at least one zeolite having channels whose pore size is defined by a ring (10MR or 12MR) having 10 or 12 oxygen atoms; preferably, the zeolite is of the ZSM-5 type.
Drawings
Figure 1 is an overall scheme of a xylene loop comprising a step of separation by adsorption, a fractionation step, a gas phase isomerization step C and a liquid phase isomerization step D.
FIG. 2a shows the distillation step B of the raffinate obtained from step A according to the prior art.
FIG. 2B is a first variant of the distillation step B of the raffinate obtained from step A, according to the invention.
FIG. 2c is a second variant of the distillation step B of the raffinate obtained from step A, according to the invention.
FIG. 3a shows the distillation step B of two raffinates obtained from step A according to the prior art.
FIG. 3B is a variant of the distillation step B of the two raffinates obtained from step A according to the invention.
Detailed Description
The characteristics and advantages of the method according to the invention will become apparent from reading the following description of non-limiting examples, with reference to the reference numerals of the embodiments illustrated in the drawings according to the invention.
For the purposes of the present invention, the various embodiments shown can be used alone or in combination with one another, without any limitation to the combination.
Accordingly, the present invention relates to a process for producing para-xylene from a feedstock containing xylene, ethylbenzene and C9+ hydrocarbons, comprising:
-a single step a of separating the feedstock, carried out in a simulated moving bed at a volume ratio of desorbent to feedstock of from 0.4 to 2.5 in a simulated moving bed separation unit at a temperature of from 20 to 250 ℃ and a pressure of from 1.0 to 2.2MPa, using zeolite as adsorbent and desorbent;
the step a may result in:
-a step B of fractionating by distillation the second fraction resulting from step a in at least one first distillation column B-C3, said step B being able to produce three fractions:
a third fraction B42 containing desorbent.
Thus, the process according to the invention makes it possible to obtain two fractions with different proportions of C8A at the end of the fractionation step, thereby increasing the PX proportion of the xylene isomerization carried out with the catalyst, operating in the liquid phase, and making it possible to limit the losses of C8A.
Simulated moving bed separation step A
According to the invention, the process comprises a single step a of separating a feedstock containing xylenes, ethylbenzene and C9+ hydrocarbons in a Simulated Moving Bed (SMB). The SMB separation step is carried out using zeolite as adsorbent and desorbent and can produce at least two fractions: fraction a1 containing a mixture of para-xylene (PX) and desorbent (also referred to as "extract"), fraction a2 containing Ethylbenzene (EB), ortho-xylene (OX), meta-xylene (MX) and desorbent (also referred to as "raffinate").
The separation step is carried out in a unit operating as a simulated moving bed in at least one separation column comprising a plurality of interconnected beds and a desorbent circulating in a closed loop, thereby obtaining two fractions:
the first fraction is extract a1, which contains, preferably consists of, para-xylene and desorbent, such that after fractionation to remove desorbent, PX reaches a minimum commercial purity of 99.0 wt%, preferably 99.9 wt%. Advantageously, extract a1 represents at least 30% by weight of the total mass of the extraction.
The second fraction is raffinate a2, which contains Ethylbenzene (EB), meta-xylene, ortho-xylene and desorbent. Preferably, the raffinate is depleted in para-xylene, i.e. the PX mass content in said raffinate is less than 1.0%, preferably less than 0.5%.
In another embodiment, at least three fractions are obtained in a single SMB separation step,
the first fraction is extract a1, which contains, preferably consists of, para-xylene and desorbent; after fractionation of the extract, commercial specification PX is obtained. Advantageously, extract a1 represents at least 30% by weight of the total mass of the extraction.
Two fractions a21 and a22 depleted in para-xylene, containing a mixture of EB, MX, OX and desorbent in variable proportions.
In this embodiment, fraction a21 and fraction a22 have different proportions of EB, MX and OX, such that the EB content of the C8A fraction of fraction a21 is greater than the EB content of the C8A fraction of fraction a 22. Preferably, PX-depleted fraction a21 contains a mixture of EB, MX, OX and desorbent; preferably, EB is the residual amount. Preferably, the EB and PX depleted fraction a22 contains a mixture of MX, OX and desorbent.
Preferably, the adsorbent used in the simulated moving bed separation unit is barium-exchanged zeolite X or potassium-exchanged zeolite Y or barium and potassium-exchanged zeolite Y.
In one embodiment, the desorbent contained in the feedstock treated via the process according to the invention is said to be heavy, i.e. it has a boiling point above that of xylene.
In another embodiment, the desorbent contained in the feedstock treated via the process according to the invention is said to be light, i.e. it has a boiling point below that of xylene.
Preferably, the desorbent used in the simulated moving bed separation unit is selected from the group consisting of para-diethylbenzene, toluene, para-difluorobenzene or diethylbenzene, alone or as a mixture.
Preferably, the volume ratio of desorbent to feed in the simulated moving bed separation unit is from 0.4 to 2.5, preferably from 0.5 to 1.5.
Preferably, the simulated moving bed separation step A is carried out at a temperature of from 20 ℃ to 250 ℃, preferably from 90 ℃ to 210 ℃, even more preferably from 160 ℃ to 200 ℃ and a pressure of from 1.0 to 2.2MPa, preferably from 1.2 to 2.0 MPa.
Preferably, the adsorber comprises a plurality of interconnected beds distributed over several zones defined by the injection of feed and desorbent, and the withdrawal of extract and raffinate. Depending on the amount of raffinate, the adsorber will preferably comprise 15-18 beds.
According to a particular embodiment, the total number of beds in the separation unit (SMB) is between 10 and 30 beds, preferably between 15 and 18 beds, distributed in one or more adsorbers, the number of beds being adjusted so that each bed has a height of between 0.70m and 1.40 m.
According to a particular embodiment (ALT0, ALT1), the distribution of the amount of adsorbent solids in each zone of the separation unit (SMB) is as follows:
the adsorbent solids amount in zone 1 is 17% ± 5%,
the amount of adsorbent solids in zone 2 is 42% ± 5%,
the adsorbent solids amount in zone 3 is 25% ± 5%,
the adsorbent solids amount in zone 4 was 17% ± 5%.
According to a particular embodiment (ALT2), the distribution of the amount of adsorbent solids in each zone of the separation unit (SMB) is as follows:
the adsorbent solids amount in zone 1 is 18% ± 8%,
the amount of adsorbent solids in zone 2 was 41% ± 8%,
the amount of adsorbent solids in zone 3A is 18% ± 8%,
the adsorbent solids amount in zone 3B is 14% ± 8%,
the adsorbent solids amount in zone 4 was 9% ± 8%.
Fractionation step B
The process according to the invention comprises a step B of fractionating by distillation a fraction a2 obtained from separation step a in at least one first three-fraction column, said fraction a2 comprising ethylbenzene, ortho-xylene, meta-xylene and desorbent. The step B may result in:
Advantageously, the first fraction B2 and the second fraction B3 comprise a mixture of EB, MX and OX in variable proportions such that the EB content in the C8A of the first fraction is greater than the EB content of the second fraction. Preferably, the difference in EB content between the first and second fractions is greater than 1.0%, preferably greater than 2.0%, preferably greater than 2.5%, preferably greater than 3.0%, preferably greater than 3.5%.
Advantageously, the three-cut column used in step B has a number of theoretical plates of from 30 to 80, preferably from 35 to 75, preferably from 40 to 80, very preferably from 45 to 65.
In one embodiment, when the second fraction B3 contains desorbent, said fraction is fed to fractionation step B in a second distillation column B-C4, thereby producing a desorbent-free, MX and OX containing fraction B31 and a desorbent consisting fraction B43.
Advantageously, when the fractionation of the raffinate is carried out in a single three-fraction column comprising 60 theoretical plates (embodiment ALT 0):
the location of the feed is on trays 30-40, and preferably on tray 37, numbered from the condenser.
The withdrawal position is located at least at the 10 th to 25 th tray above the feed, preferably at the 18 th tray above the feed.
Advantageously, when the fractionation of the raffinate is carried out in a first three-cut column and then in a second two-cut column, each column preferably comprises 47 theoretical plates (embodiment ALT1, embodiment ALT 2):
the location of the feed is on trays 16 to 24, and preferably between trays 18 to 22, numbered from the condenser.
The point of withdrawal from the three-fraction column is between the 5 th and 10 th trays below the feed point, preferably between the 7 th and 8 th trays below the feed.
The position of withdrawal and of the feed of the raffinate column can be similarly adjusted as a function of the total number of trays installed in each raffinate column.
Preferably, fraction a1 obtained from step a containing a mixture of PX and desorbent, preferably consisting of a mixture of PX and desorbent, is sent to a step of fractionation by distillation in a distillation column (B-C1) to produce a fraction B1 consisting of PX and a fraction B41 consisting of desorbent. The distillation is carried out according to the knowledge of the person skilled in the art.
In a particular embodiment of the invention (fig. 2B, ALT0), fraction a2 obtained from step a is fed to a step of fractionation by distillation in a first three-fraction distillation column B-C3, resulting in a fraction B2 containing EB, MX and OX at the top, a fraction B3 free of desorbent and containing OX and MX, preferably MX and OX, and residual amounts of EB, being produced as side draw, and a fraction B42 containing desorbent, preferably consisting of desorbent, at the bottom.
In a preferred embodiment (fig. 2C, ALT1), the three-cut column B-C3 is used to:
a first fraction B2 is obtained at the top of the column, which contains, preferably consists of, EB, OX and MX,
a second fraction B3 is obtained as a side draw, which contains OX, MX, desorbent and a residual amount of EB, preferably consisting of OX, MX and desorbent, and
a fraction B42 containing a desorbent, preferably consisting of a desorbent, is obtained at the bottom of the column.
Said fraction B3 is fed to a second two-fraction distillation column B-C4, resulting in a desorbent-free, MX-and OX-containing fraction B31 and a desorbent-composed fraction B43.
Advantageously, embodiment ALT1 (fig. 2c) can reduce xylene content in EB-rich fraction B2.
In another preferred embodiment ALT2, step a allows the production of two fractions a21 and a22 (fig. 3b, ALT 2). Said fraction A21 is sent to step B of fractionation by distillation in a first three-fraction distillation column B-C3, so that:
a first fraction B2 containing EB, OX and MX, preferably consisting of EB, OX and MX, is produced at the top of the column,
a second fraction B3 containing OX, MX, desorbent and optionally a residual amount of EB, preferably consisting of OX, MX and desorbent, is produced as a side draw, and
a third fraction B42 containing, preferably consisting of, a desorbent is produced at the bottom of the column.
In embodiment ALT2, said second fraction B3 obtained from the first column used in step B and fraction a22 obtained from step a are fed to a second distillation column B-C4, thereby producing a fraction B31 containing MX, OX and optionally residual amounts of EB, preferably consisting of OX and MX, and a fraction B43 containing desorbent, preferably consisting of desorbent.
Preferably, fraction A22 is introduced into the second column of the raffinate, preferably the two-fraction column B-C4, below the injection point of fraction B3.
Advantageously, in the embodiment ALT2 (fig. 3B), three-cut column B-C3 receives only EB-rich fraction a21 harvested from raffinate a2 of step a by SMB separation, which may further reduce xylene content in EB-rich fraction B2 relative to embodiment ALT 1.
Advantageously, the desorbent fraction B41, fraction B42 and fraction B43 free of C8A are recovered at the bottom of each distillation column, combined into fraction B4 and sent to simulated moving bed adsorption step a.
When the desorbent of step A is a compound heavier than xylene, i.e., a compound boiling above the boiling point of xylene, an EB-rich raffinate is produced overhead of the three-cut column, and a xylene-rich raffinate is obtained in the side draw or overhead of the second distillation column.
When the desorbent of the separation step is a compound lighter than xylene, i.e. a compound having a boiling point lower than that of xylene, a xylene-rich stream is produced at the bottom of the three-cut column, an EB-rich raffinate is obtained in the side draw or at the bottom of the second distillation column, and a desorbent free of C8A is withdrawn at the top of the two columns and recycled to the adsorption step.
Advantageously, the distillation columns B-C1, B-C3 and B-C4 used in the fractionation step B are operated at atmospheric pressure, the reboiling temperature is 210-.
Advantageously, the raffinate B3 recovered overhead in column B-C3 is obtained in the form of a liquid distillate withdrawn at the fourth tray below the condenser, which is at total reflux and comprises a decantation tank to withdraw liquid water.
More precisely, when step B uses a distillation column (B-C3), the performance of the fractionation steps (ALT0 and ALT1) of the raffinate obtained by distillation is characterized by:
recovery of EB (DR) in EB-rich raffinate B2: dr (EB) = EB(raffinate B2)/(EB(raffinate B2)+ EB(raffinate B3))
Recovery of xylenes (DR) in EB-rich raffinate B2: dr (XYL) = XYL(raffinate B2)/(XYL(raffinate B2)+ XYL(raffinate B3))
When step B uses a second distillation column (B-C4), the performance of the fractionation step (ALT2) of the raffinate obtained by distillation is characterized by:
recovery of EB (DR) in EB-rich raffinate B2: dr (EB) = EB(raffinate B2)/(EB(raffinate B2)+ EB(raffinate B31))
Recovery of xylenes (DR) in EB-rich raffinate B2: dr (XYL) = XYL(raffinate B2)/(XYL(raffinate B2)+ XYL(raffinate B31))
Advantageously, in the process according to the invention, the recovery of ethylbenzene (denoted dr (eb)) is between 50% and 90%, preferably between 80% and 90%.
Advantageously, in the process according to the invention (ALT0 or ALT1), when fraction a1 and fraction a2 are sent to the fractionation step, the recovery of xylenes dr (xyl) differs with respect to the recovery of ethylbenzene (dr (eb)) by at least 2% less, preferably by at least 5% less, preferably by at least 10% less, preferably by at least 15% less, than the former.
Advantageously, the recovery of xylenes, dr (xyl), shows a strictly at least 20% less difference, preferably at least 23% less difference when fraction a1, fraction a21 and fraction a22 are sent to the fractionation step.
According to another embodiment, the distillation step may be carried out using any other distillation arrangement including first three-fraction distillation column B-C3. Preferably, the three-cut distillation column B-C3 according to variant ALT0, ALT1 or ALT2 of the present invention comprises internal walls to improve the performance in separating the two raffinate B2, B3 without desorbent.
Gas phase isomerization step C
Advantageously, the process comprises a step C of gas-phase isomerization of the raffinate B2 containing ethylbenzene, ortho-xylene and meta-xylene obtained from fractionation step B.
Advantageously, the gas phase isomerization step allows to isomerize OX and MX as well as EB in a unit operating in the gas phase at high temperature and converting ethylbenzene into xylenes, to treat the EB rich raffinate B2 obtained from step B.
Preferably, the mass ratio of raffinate B2 fed to isomerization step C to the total raffinate obtained at the end of step B (B2+ B3 or B2+ B31) is between 20% and 90%, preferably between 25% and 60%, more preferably between 30% and 45%. The ratio advantageously maximizes the production of para-xylene. When the ratio is high, the PX production can be increased without any compromise in the capacity of the paraxylene loop; when the ratio is moderate, the PX production increases more dramatically, but with a slight increase in xylene ring production.
One advantage of the process according to the invention is that the fixed concentration of EB in the xylene loop can be increased, which can increase EB conversion by entering the first gas phase isomerization unit.
Thus, the combination of the step of separation by adsorption in SMB with the step B of fractionation by distillation in a three-fraction column and two isomerization steps can improve the overall yield of para-xylene of the aromatic loop and minimize the economic impact.
According to the present invention, the gas phase isomerization step can convert EB to xylenes with a conversion per ethylbenzene pass (run) of generally 10% to 50%, preferably 20% to 40%, with a loss of C8 aromatics (C8A) of less than 5.0 wt%, preferably less than 3.0 wt%, preferably less than 1.8 wt%.
At a temperature above 300 ℃, preferably 350 ℃ to 480 ℃, a pressure of less than 4.0MPa, preferably 0.5 to 2.0MPa, for less than 10.0h-1Preferably 0.5 h-1To 6.0h-1A hydrogen to hydrocarbon molar ratio of less than 10.0, preferably from 3.0 to 6.0, and in the presence of a catalyst comprising at least one zeolite having channels whose pore size is defined by a ring (10MR or 12MR) having 10 or 12 oxygen atoms and at least one metal from group VIII in an amount of from 0.1% to 0.3% by weight.
Any catalyst (which may or may not be based on zeolite) capable of isomerizing hydrocarbons containing 8 carbon atoms is suitable for use in the gas phase isomerization unit. Preferably, the catalyst used comprises an acidic zeolite, such as a zeolite of the MFI, MOR, MAZ, FAU and/or EUO structure type. Even more preferably, the catalyst used comprises a zeolite of structure type EUO and at least one metal from group VIII of the periodic Table of the elements.
According to one preferred variant of the process, the catalyst used in step C comprises from 1% to 70% by weight of a zeolite of structure type EUO (preferably EU-1), comprising silicon and at least one element T preferably selected from aluminium and boron, the zeolite having a Si/T ratio ranging from 5 to 100.
Preferably, the zeolite is at least partially in the hydrogen form and the sodium content is such that the Na/T atomic ratio is less than 0.1.
Preferably, the catalyst comprises from 0.01% to 2% by weight of tin or indium and a proportion of sulphur of from 0.5 to 2 atoms per atom of metal from group VIII.
The effluent C1 obtained in step C having a PX, OX and MX isomer concentration close to the thermodynamic equilibrium concentration is recycled to the simulated moving bed adsorption step a.
In a particular embodiment, when the effluent C1 contains heavy compounds and light compounds formed via undesired reactions, the effluent is then sent to an optional fractionation step to remove the compounds.
Liquid phase isomerization step D
Advantageously, the process comprises a step D of liquid phase isomerization of fraction B3 or fraction B31 containing ortho-xylene and meta-xylene obtained from fractionation step B.
Thus, the process according to the invention makes it possible to increase the proportion of xylene isomerization carried out by the catalyst operating in the liquid phase and producing the lowest losses of C8A.
Xylene isomerization step D is carried out in the liquid phase with EB conversion per pass less than or equal to 5.0%, preferably less than or equal to 3.0%, and preferably less than or equal to 0.2% and achieves isomerization of the xylene mixture such that PX approaches a thermodynamic equilibrium greater than or equal to 90.0%, preferably greater than or equal to 94.0%.
At a temperature below 300 ℃, preferably 200 ℃ and 260 ℃, a pressure below 4.0MPa, preferably 1.0-3.0MPa, for less than 5.0h-1Preferably 2.0 to 4.0h-1And carrying out the liquid phase isomerization step D in the presence of a catalyst comprising at least one zeolite with channels having a pore size defined by a ring having 10 or 12 oxygen atoms (10MR or 12MR), preferably the catalyst comprises at least one zeolite with channels having a pore size defined by a ring having 10 oxygen atoms (10MR), even more preferably the catalyst comprises a ZSM-5 type zeolite.
The patent US 8697929 describes in more detail the operating conditions and the gas phase isomerization catalyst and the liquid phase isomerization catalyst that can be used in the process according to the invention.
Advantageously, the effluent D1 obtained in step D, having a PX, OX and MX isomer concentration close to the thermodynamic equilibrium concentration, is recycled to the simulated moving bed adsorption step a.
In a particular embodiment, when the effluent D1 contains heavy compounds and light compounds formed via undesired reactions, the effluent is then sent to an optional fractionation step to remove the compounds.
Detailed Description
The following examples illustrate the invention without limiting its scope.
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